The presence of local electric discharges, so-called partial discharges, in the insulation system for electric installation components (hereinafter referred to as "components" only) or electric high-voltage apparatus, such as, for example, instrument transformers, switching devices, generators, power transformers, cables, etc., constitutes an important parameter when assessing the quality, condition, and expected service life of the component. Therefore, the acceptance tests of components for an electric high-voltage installation normally comprise a testing with respect to the occurrence of partial discharges under well defined conditions, for example according to IEC Publ. 270 "Partial Discharge Measurements". In addition to this, different methods and devices have been developed for testing at the site of the installation for the purpose of estimating, after some time in operation, the condition of components and hence, for example, creating a more reliable basis for decision on preventive maintenance. For power generators, for example, measurement equipment is also available which allows a continuous monitoring during operation.
In installations and/or components which comprise several electric circuits, it is also important, in addition to being able to distinguish partial discharges from external noise, to be able to locate a detected partial discharge to a definite electric circuit, for example a certain generator coil in a power generator.
A partial discharge in a component gives rise to current pulses through the conductors by which the component is connected to the surroundings and these current pulses can be sensed either by measuring the voltage caused by the current pulses when passing through a measuring impedance connected to one of the conductors, or by a direct or indirect current measurement. During voltage measurement, the measurement equipment comprises a coupling capacitor, which must be free from partial discharges under the conditions wherein the measurement is carried out, and the test object, the coupling capacitor and the measuring impedance are galvanically interconnected, in a known manner, and connected to the surrounding components and testing equipment, respectively.
One problem when measuring partial discharges is to distinguish measurement signals emanating from partial discharges in the test object from disturbances emanating from partial discharges in the surrounding components or in the testing equipment or which have another origin but a frequency and amplitude spectrum resembling the spectrum of the partial discharges. Such disturbances may be passed to the test object through the conductors by which they are galvanically connected to the surroundings but may also, in the frequency ranges used, be captured by the measurement device by electromagnetic radiation from the surroundings.
During testing in a test room environment, the noise level may be reduced by screening the test room and filtering the voltage supply, but this is normally not possible when testing at an installation site. Disturbances of the kind mentioned may also be suppressed by so-called balanced test circuits where the component to be tested is balanced against a reference impedance, in which case disturbances occur as cophasal signals across measuring impedances series-connected to the test object and to the reference impedance. The reference impedance may comprise a component similar to the test object or of an impedance imitating this, usually a reference capacitor, which is then to be free from partial discharges during the testing. The balanced test circuits may also be designed as complete bridge connections. The above-mentioned coupling capacitors and the reference impedances and measuring impedances, respectively, must be galvanically connected to the component during testing and also to the high-voltage source to which the test object is connected.
Voltage measurement across a measuring impedance has the fundamental disadvantage that the measurement sensitivity decreases with increasing capacitance of the test object.
When sensing the partial discharge by current measurement, only one sensor is required, which is sensitive to the magnetic field associated with the current pulse. The measurement signal obtained from such sensors may thus be kept galvanically separated from the test object and the high-voltage circuit connected to the test object, thus eliminating problems such as the influence from loops in the ground circuits. Current-measuring sensors eliminate the need for coupling capacitors and measuring impedances and can be designed with very small dimensions, which makes it simple to arrange them at components of varying dimensions and shapes.
German patent DE 37 08 731 describes an electric switching device for detecting interference pulses, particularly in partial discharges, in a high-voltage installation. Between a live part of the installation and ground potential, a voltage divider of a capacitive nature is connected. The voltage from the medium voltage terminal thereof is supplied, via impedance networks adapted for the purpose, to a circuit for evaluating partial discharges. Such a device cannot usually distinguish pulses emanating from partial discharges in the component from interference pulses originating from outside the component in question, and the device shown in such patent document suggests a method of dealing with this problem. A current transformer is connected with its primary winding to a conductor which, at high-voltage level, connects the component to the surroundings, and its secondary winding is divided into two parts by means of a center tap, each part being loaded by a resistance.
The secondary winding is wound in such a way that the voltages between the center tap and the respective terminal are equal in magnitude but of opposite phase position. The medium-voltage terminals of the voltage divider are connected to the center tap on the secondary winding of the current transformer so that the voltage at the respective terminal of the secondary winding consists of the vectoral sum of the voltage from the voltage divider and the voltage across the corresponding part of the secondary winding. Depending on the current direction through the primary winding of the current transformer, different magnitudes of these voltages are thus obtained. A partial discharge within the component gives rise to a current pulse through the primary winding of the current transformer in a direction away from the component towards its surroundings whereas an externally arriving interference pulse gives rise to an interference pulse through the primary winding of the current transformer in the opposite direction. By evaluating the voltages sensed at the terminals of the secondary winding, it can thus be determined whether a detected pulse emanates from a partial discharge in the component in question.
The current transformer may advantageously be designed as a bifilarly wound Rogowski coil. In components with a plurality of voltage terminals, a more accurate location of a partial discharge may be achieved by arranging the switching device at more than one voltage terminal. The measurement principle is thus based on a voltage proportional to the voltage across the component being available and therefore comprises, in addition to a device for direction-dependent current sensing, also a voltage divider of a high-pass character. In the event that such a voltage divider cannot be galvanically connected at the component in question, the use of a field probe or an aerial for capacitive sensing of the voltage on the component is proposed.
European patent EP 0 061 254 B describes a device for monitoring of partial discharges in high-voltage equipment comprising more than two components and exhibiting more than two discharge paths for a partial discharge, for example a transformer substation, in which case it should be possible to individually, and continuously, monitor each selected component and to distinguish partial discharges from noise and signals emanating from externally arriving voltage transients.
The described device is designed for monitoring a number of components in the form of high-voltage devices, connected between a common high-voltage busbar and ground potential or a low-voltage conductor. At their connection to ground potential and to the low-voltage divider, respectively, the components to be monitored have a discharge detection unit inductively connected to the connection by means of a high-frequency current transformer. A partial discharge in a component gives rise to a current pulse through the current transformer belonging to the component but, since the components, the high-voltage busbar and ground potential and the low-voltage divider, respectively, form a network for the partial discharge paths, also through the other current transformers, but with reversed polarity therein. A voltage transient or other disturbance on the high-voltage busbar, on the other hand, gives rise to current pulses of the same polarity through all the current transformers. Each of the discharge detectors delivers a coded signal, the pulse width of which corresponds to the polarity of the detected current pulse, to a decoder. The decoder comprises a number of demultiplexers, which generate and store a signal pattern corresponding to the detected state of polarities for each of the components. This signal pattern is decoded and evaluated, whereby a deviating polarity of a detected current pulse from one component indicates a partial discharge therein.
The device requires an extensive electronic system for coding and evaluation, comprising monostable multivibrators, memory and decoding circuits, and the system in its entirety is necessarily given a large physical extension. A pulse emanating from a partial discharge in a component is damped during its propagation along the busbar and through the other components, which may result in not all the discharge detection units delivering a signal to the demultiplexers. The device described above will thus not function as intended and a special electronic circuit has been introduced to indicate and signal this state. It is also conceivable that more than one of the discharge detection units deliver a signal with a polarity deviating from that of the remainder, which may be due to correctly or incorrectly detected partial discharged. Also this results in the device not functioning in the intended way and also this state is sensed and signalled by means of a special electronic circuit.